Abstract
Transition-metal dichalcogenide (TMD) semiconductors have attracted interest as photoelectrochemical (PEC) electrodes due to their novel band-gap structures, optoelectronic properties, and photocatalytic activities. However, the photo-harvesting efficiency still requires improvement. In this study, A TMD stacked heterojunction structure was adopted to further enhance the performance of the PEC cathode. A P-type WSe2 and an N-type MoS2 monolayer were stacked layer-by-layer to build a ultrathin vertical heterojunction using a micro-fabrication method. In situ measurement was employed to characterize the intrinsic PEC performance on a single-sheet heterostructure. Benefitting from its built-in electric field and type II band alignment, the MoS2/WSe2 bilayer heterojunction exhibited exceptional photocatalytic activity and a high incident photo-to-current conversion efficiency (IPCE). Comparing with the monolayer WSe2 cathode, the PEC current and the IPCE of the bilayer heterojunction increased by a factor of 5.6 and enhanced 50%, respectively. The intriguing performance renders the MoS2/WSe2 heterojunction attractive for application in high-performance PEC water splitting.
Highlights
Hydrogen-based energy is a clean, sustainable, and highly efficient energy resource
When the thickness of Transition-metal dichalcogenide (TMD) is varied from bulk to single layer, which become two-dimensional (2D) materials, their band gaps change from indirect (1.0–1.6 eV) to direct (1.6–1.8 eV) [7]. 2D material is a kind of layered material that consists of single or few atomic layers, such as graphene
The atomic force microscopy (AFM) showed that the step heights of monolayer WSe2 on the Au electrode and that of monolayer MoS2 on WSe2 were 0.7 and 0.8 nm, respectively, which confirmed the monolayer thickness of the WSe2 and MoS2 sheet (Fig. 2b)
Summary
Hydrogen-based energy is a clean, sustainable, and highly efficient energy resource. Intensive research has been conducted to realize efficient production of hydrogen, and photoelectrochemical (PEC) water splitting is considered as a promising method [1,2,3,4,5]. As a PEC cathode, the advantages of the TMD heterojunction are that: (1) the built-in field in the depletion layer of the p–n junction may accelerate separation of the photo-generated excitons, as well as restrict recombination of the electron–hole pair to improve the PEC performance [26,27,28]; (2) the atom-thin vertical heterojunction could shorten the diffusion distance and rapidly deliver the excitons to the solid–liquid interface for redox reaction [17, 29]; (3) due to the large contact area in the heterojunction, more charge could be efficiently separated simultaneously; (4) an extended region of the visible-light spectrum could be utilized by this MoS2/WSe2 heterojunction. The mechanism of enhancement of the PEC characteristics of the 2D heterojunction is discussed
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